1. Field of the Invention
The present invention relates to an apparatus for communication devices, in particular an apparatus for providing a constant current while the device is in the xe2x80x9coff-hookxe2x80x9d state.
2. Description of the Related Art
Modems are communications devices which employ digital processing techniques to transmit data over a band-limited communications channel, such as the Public Switched Telephone Network (PSTN). Generally, to transmit data over the PSTN, a modem modulates the binary data and then transmits the data over the band-limited communications channel. A receiving modem is coupled to the band-limited communications channel, and receives the binary data and demodulates the binary data
Generally, in order for the modem to effectively communicate over the PSTN, such as a Plain Old Telephone Service (POTS) line, the modem""s xe2x80x9coff-hookxe2x80x9d electrical interface should meet a telephone standard, such as EIA/TIA-496-A. The standard includes the necessary electrical interface criteria for the modem, including xe2x80x9coff-hookxe2x80x9d impedance, loop current, and dial tone levels.
xe2x80x9cOff-hookxe2x80x9d refers to a state of the communication device, such as when the communication link between the device and the PSTN is enabled for voice, data communication or network signaling. The term xe2x80x9coff-bookxe2x80x9d is derived from the original telephone usage in which they refer to the position of the hand set with respect to the cradle of the telephone. An improper interface between the device and POTS line may result in lost data. In addition, a variety of other performance criteria, such as connection speed of the device, may be adversely affected by an improperly interfaced device. Furthermore, an improper device interface may degrade POTS network performance.
Present modem designs incorporate a fixed valued resistor or a resistor/transistor element as a current element when the modem is in the xe2x80x9coff-hookxe2x80x9d state. This off-hook impedance is placed in series with the telephone line impedance. Since most modems use an amplitude modulation scheme for data transmission, DC biasing currents can affect modem communication. Furthermore, because telephone line impedance can vary, data transmission can be affected by the present modem designs utilizing a fixed-value resistor design.
Briefly, one feature of the invention provides a constant current to an Analog Front End (AFE) of a communication device while the device is in the off-hook state. The apparatus provides the necessary circuitry for connecting a communications device, such as a modem, with a telephone network, such a Plain Old Telephone Service (POTS) network. When the communication device is in an off-hook state, the apparatus provides a constant DC current to the telephone line. Transmission and reception and of data over the telephone network is optimized when the average DC biasing current of a data signal is about 30 ma. The apparatus generally maintains an average current of about 30 ma regardless of the loading of the telephone network line. Since most communication devices use current modulation for data transmission, the present invention provides a constant DC biasing current for such data transmission.